Re: [802.3_EPOC] Why multiple simultaneous MCS
Victor,
I assume here you're talking about absolute need as in absolute need in a real time fashion. I would expect an operator would want to have the ability to see at which sub-channels are operating well and which are not. We will also need to allow operators to restrict the use of some sub-channels. I believe someone mentioned the FCC being involved in prohibiting use of certain frequencies in this or another thread.
I agree with you that the real time information needed by the upper layers is indeed a very limited set. But the timing of that information transfer is crucial to proper operation. If an upper layer scheduler finds out that the channel data rate to a given LLID increased some time ago, that is fine. If on the other hand it finds out that an LLIDs data rate decreased 50 ms ago that may be a very different matter. This is one of the reasons I cringe when some says that the PHY can act autonomously in setting up the channels.
Best Regards,
Duane
FutureWei Technologies Inc.
duane.remein@xxxxxxxxxx
Director, Access R&D
919 418 4741
Raleigh, NC
From: Victor Blake [mailto:victorblake@xxxxxxxxxxxxxxx]
Sent: Thursday, October 11, 2012 7:41 PM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Matt,
OK, yes I guess I did mistakenly (my mistake) have that impression. So I think one thing to reasonably ask is, for whatever happens below the MAC, what of that information has to be elevated above the MAC, not in protocol operations, but administratively. For example, if, after the physical layers are "linked up" (I'd call that the physical link layer) and the MAC is running, we have DSx/USy Mbps available, how do we convey that northbound to be used for management/operations. Obviously the CLT can have some mechanisms (as an IP addressable or by proxy from the OLT - I'm not sure) to be queried for or to push that information. But the real question might be, before forwarding begins, what does MPCP at the OLT ABSOLUSTELY NEED to know ? I understand it needs to know data rates. It isn't clear to me that it needs to know anything else at all other than the data rates available.
Not responding to you Matt, but in the previous discussion ...
On multicast, I think we can make (by decision) all of our lives easy, but making an assumption, and sticking to it, that multicast is broadcast on the entire EPON and passively forwarded through all CLT. I don't see how anything that happens in terms of that phy setup would prevent that from implicitly working. Again, if CLT is not a bridge (MAC transparent) and certainly isn't a router or better stated - not a multicast node, then everything broadcast on the EPON MUST be forwarded past the CLT. So I'm not sure I following what the questions/discussion is around multicast/video with EPoC other than, forward all broadcast/multicast as MUST.
-Victor
From: Matthew Schmitt [mailto:m.schmitt@xxxxxxxxxxxxx]
Sent: Thursday, October 11, 2012 5:25 PM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Victor,
I agree that doing things above the MAC is outside the charter, so I definitely wasn't implying that we should go that road (and I hope no one got that impression). The MAC Control Sub-layer is definitely in scope to modify if needed to support EPoC, although we want to minimize what we touch there, and ideally to do so in such a way as to not break or cause problems with EPON devices.
Thanks.
Matt
From: Victor Blake <victorblake@xxxxxxxxxxxxxxx<mailto:victorblake@xxxxxxxxxxxxxxx>>
Date: Thursday, October 11, 2012 1:38 PM
To: Matthew Schmitt <m.schmitt@xxxxxxxxxxxxx<mailto:m.schmitt@xxxxxxxxxxxxx>>, "STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>" <STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>>
Subject: RE: [802.3_EPOC] Why multiple simultaneous MCS
Matt,
Can I ask, instead of pondering why we might do this or that above the EPON MAC, why not just decide - consistent with the project charter, that we are not going to touch (other than management) anything above the MAC and then focus on solving the problems below the MAC ? All of these problems can in fact be solved below the MAC. It isn't really a matter of whether it is possible, just whether we spend time doing it or time exploring changing the MAC which defeats the point of EPOC.
It's not that I disagree that we couldn't use higher layers to do certain functions, it's that doing so breaks with the PAR and will lead to a dead end without the goals being achieved.
-Victor
From: Matthew Schmitt [mailto:m.schmitt@xxxxxxxxxxxxx]
Sent: Thursday, October 11, 2012 3:13 PM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Eugene,
One thing I wonder about is just how much we're going to have to modify the EPoC system relative to EPON, regardless of this particular potential feature. That could result in a somewhat different answer regarding how much of a change this would take. I think the choice of how to implement it can significant impact how much of a change as well (such as where decisions are made in the stack). Which is why I agreed to (more or less) let go of the argument for now, pending a more concrete proposal that can undergo more thorough analysis. :-)
Thanks.
Matt
From: <Dai>, "Eugene (CCI-Atlanta)" <Eugene.Dai@xxxxxxx<mailto:Eugene.Dai@xxxxxxx>>
Date: Thursday, October 11, 2012 12:07 PM
To: Matthew Schmitt <m.schmitt@xxxxxxxxxxxxx<mailto:m.schmitt@xxxxxxxxxxxxx>>, "STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>" <STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>>
Subject: RE: [802.3_EPOC] Why multiple simultaneous MCS
Matt: This is how D3.1 supposed to work; but not without significant change in EPON system.
Eugene Dai PhD
Principle Transport Architect
COX Communications
Tel: 404-269-8014
From: Matthew Schmitt [mailto:m.schmitt@xxxxxxxxxxxxx]
Sent: Thursday, October 11, 2012 11:55 AM
To: Dai, Eugene (CCI-Atlanta); STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Eugene,
Alternately, you could send the multicast (and MPCP messages) on a profile that everyone listens to and is set to the lowest common denominator.
Thanks.
Matt
From: <Dai>, "Eugene (CCI-Atlanta)" <Eugene.Dai@xxxxxxx<mailto:Eugene.Dai@xxxxxxx>>
Reply-To: "Dai, Eugene (CCI-Atlanta)" <Eugene.Dai@xxxxxxx<mailto:Eugene.Dai@xxxxxxx>>
Date: Thursday, October 11, 2012 9:45 AM
To: "STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>" <STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>>
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Hi Victor: We haven't met, but I know your name from DPOE.
Your comments on how EPON handle multicast video are absolutely correct; but that is also exactly the problem for multi MCS profiles for EPOC PHY. The multiple MCS PHY profiles are essentially creating multiple small pipes inside what you referred as "big data pipe" at PHY layer. As we know that EPON DS is broadcast in nature, it is very efficient in handling multicast video as you described. With this multiple PHY profiles we have duplicate each broadcast and multicast streams to each of the small pipes, including EPON MAC messages.
Eugene Dai PhD
Principle Transport Architect
COX Communications
Tel: 404-269-8014
From: Victor Blake [mailto:victorblake@xxxxxxxxxxxxxxx]
Sent: Thursday, October 11, 2012 11:26 AM
To: Dai, Eugene (CCI-Atlanta); STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: RE: [802.3_EPOC] Why multiple simultaneous MCS
Hi Eugene,
I don't think we've met, but I've been following your comments with interest.
I don't see the relationship to "multicast video." Unlike HFC today, the goal is not to FDM video and data, it's one big fat data pipe which is bandwidth managed by IP and Ethernet. Not that it's at all related to EPoC (because I don't think it is), but one strict fix for that is simply an LLID for video. That would all happen above the PHY without respect to other than the obvious fact that one cannot administrative configure an LLID to support more bandwidth than the PHY supports and that obviously one must do calculations (either in engineering or admistratively on the box - again without reference to the protocol) as to how to share the shared bandwidth (aka configuration management).
Victor R. Blake
Independent Consultant
victorblake@xxxxxxxxxxxxxxx<mailto:victorblake@xxxxxxxxxxxxxxx>
http://www.victorblake.com<http://www.victorblake.com/>
http://twitter.com/victorblake
540-338-1076
From: Dai, Eugene (CCI-Atlanta) [mailto:Eugene.Dai@xxxxxxx]
Sent: Thursday, October 11, 2012 10:38 AM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Duane: I suggest that we consider some realistic examples, instead of this kind of wild argument. All DOCSIS CM support 256QAM today, some may support 1024 QAM in the future. Assuming ~100MHz DS bandwidth allocation for EPOC, with 1024 QAM we get 1 Gbps raw bandwidth, with 256 QAM the raw bandwidth will be 800 Mbps. Where is 250Mbps in your example come from?
We can make MAC to aware PHY or more preciously PHY rate, but what price we have to pay? The impacts are not limited at PHY layer; it will propagate to system level, for example how to deal with multicast video? Unnecessarily duplicate multicast video streams to different PHY profiles could consume more bandwidth than possible gain. Backward compatibility is another concern.
The ultimate goal is to improve outside plant condition to accommodate a reasonably higher modulation orders.
Eugene Dai PhD
Principle Transport Architect
COX Communications
Tel: 404-269-8014
From: Duane Remein [mailto:Duane.Remein@xxxxxxxxxx]
Sent: Wednesday, October 10, 2012 9:51 PM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Marek,
A simple example.
An EPoC network with 10 CNUs, 9 can operate at 1G, 1 can only attain 250M
With 2 MCS profiles you can attain an average instantaneous network data rate of 950 Mbps or 95 Mbps per user.
With LCD you can only attain an average instantaneous network data rate of 250 Mps or 25 Mbps per user.
Best Regards,
Duane
FutureWei Technologies Inc.
duane.remein@xxxxxxxxxx<mailto:duane.remein@xxxxxxxxxx>
Director, Access R&D
919 418 4741
Raleigh, NC
From: Marek Hajduczenia [mailto:marek.hajduczenia@xxxxxx]
Sent: Wednesday, October 10, 2012 6:20 PM
To: Duane Remein; STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: RE: [802.3_EPOC] Why multiple simultaneous MCS
Duane,
Could you please kindly point to me to any study which shows that "significantly lower" bandwidth ? I failed to find any such contribution submitted to EPoC TF / SG. Perhaps I missed something along the way.
Marek
From: Duane Remein [mailto:Duane.Remein@xxxxxxxxxx]
Sent: Wednesday, October 10, 2012 16:23
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Matt,
I suspect another argument for multiple profiles is that you can offer a higher level of service. Like it or not, networks are assessed based on the average sustained data rate that can be delivered to a customer. With a lowest common denominator approach this would be significantly lower.
Best Regards,
Duane
FutureWei Technologies Inc.
duane.remein@xxxxxxxxxx<mailto:duane.remein@xxxxxxxxxx>
Director, Access R&D
919 418 4741
Raleigh, NC
From: Matthew Schmitt [mailto:m.schmitt@xxxxxxxxxxxxx]
Sent: Tuesday, October 09, 2012 11:25 PM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: [802.3_EPOC] Why multiple simultaneous MCS
Based on some of the discussions, I think it might be worthwhile to spend a moment going into the chain of reasoning that led myself (and others) to the conclusion that an advanced next generation HSD system for HFC (such as EPoC) should include the ability to support multiple simultaneous Modulation and Coding Schemes (MCS).
If I were to sum it up in 2 words (okay, technically 4), it would be this: SNR Headroom.
In a system that requires all end stations to operate with the same MCS (and to receive all downstream transmissions), the great danger is that for whatever reason the SNR to a given end station will change sufficiently that it will drop offline. That simply cannot be allowed to happen. As a result, operators are required to maintain a significant amount of "SNR Headroom" when they setup and deploy an end station. Common practice is to allow on order of 8 db of SNR headroom when deploying a modem (some a little more, but I'm not aware of any that do less).
What this means is the following... Let's say that you needed 25 dB of SNR to operate 1024 QAM with a 3/4 LDPC FEC code (which I believe is the case with DVB-C2, and serves as a useful reference point here). An MSO would not actually use that MCS unless they could get their end stations at 33 dB of SNR (25+8) when deploying them. If they couldn't get to that level when deploying the end station, they wouldn't be able to use that MCS.
I pulled out that particular number because, based on the SNR distributions that were shared previously, even if you assumed you could "bring up" the bottom 2.1% of modems, that's the best you could expect to do. And even if you could further improve SNR, you're still always going to be operating in effect 8 dB below what you could theoretically do. For example, just to get to 4096 QAM with a 5/6 LDPC FEC, you'd need to get every single end station to operate at about 40dB or higher of SNR.
Now, if you could get away from requiring that 8 dB of headroom, it would be huge, considering that 6 dB is an order of modulation with the same FEC, an increase of 2 bits per symbol.
One way to achieve that end is to support multiple MCSs, but only one at a time, which is set based on the "lowest common denominator". In this approach, if an end station were to have a sudden drop in SNR, then the system would need to drop to a lower "lowest common denominator", until such time as the problem could be addressed. By having that fall back, you could in theory operate closer to the theoretical maximum and reduce the amount of headroom required.
However, there are some issues here. First, one bad end station coming onto the network - or a single problem in a single household - would affect the entire network, degrading the service of every single end station. Second, particularly in the scenario where an end station encounters a problem after it's already online, there's no guarantee that it would even still be able to communicate that it was having a problem and that it needed the MCS to be changed. That risk is then going to force you to leave more headroom, mitigating any potential gains.
The only approach that we've come up with so far that allows you to significantly reduce that headroom and operate close to theoretical levels - while not adversely affecting the entire system when there's a problem - is to create a system in which a single end station is able to fall back to a more robust MCS if it encounters a problem. In that way, only the single device having an issue is affected, and unless the problem is truly catastrophic you can reliably keep the mode online (meaning that you have an opportunity to fix the problem while the customer remains online and operational).
We could argue back and forth for quite a while about SNR spreads on the network, how much variation there will be between end stations in a given service group, etc. And the ability to operate each device to its maximum potential is very appealing to be sure. But what really makes this decision, in my opinion, is the SNR headroom issue, and in particular the ability to significantly reduce it while actually improving robustness and the ability to keep customers on line.
Thanks.
Matt
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